This application claims the priority of Korean Patent Application No. 2004-3801, filed on Jan. 19, 2004 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a cathode active material for a lithium rechargeable battery, and more particularly to a cathode active material for a lithium rechargeable battery which can reduce the expansion rate of batteries at high temperatures, and a lithium rechargeable battery using the same.
2. Description of the Related Art
The need for lightweight high performance portable electronic devices such as camcorders, mobile phones and laptop computers, increased the amount of research into batteries that are a power source for those devices. In particular, rechargeable lithium rechargeable batteries have 3 times the energy density per unit weight as Pb storage batteries, Ni—Cd batteries, Ni—H batteries, and Ni—Zn batteries. In addition, rechargeable lithium rechargeable batteries can be charged rapidly. These advantages make the extensive use of rechargeable lithium rechargeable batteries more promising hence, research and development of this technology is increasing.
In lithium rechargeable batteries, the cathode is composed of a composite oxide of lithium and transition metals, such as LiCoO2, LiNiO2, LiMnO2, LiMn2O4, LiFePO4, LiNixCo1−xO2, and LiNi1−x−yCoxMnyO2. The anode is composed of lithium metal, lithium metal alloy, carbon-based materials, or graphite-based materials. If the anode of a lithium battery is composed of a lithium metal or a lithium metal alloy, the battery is referred to as a lithium metal battery. If an anode of a lithium battery is composed of carbon-based materials the battery is referred to as a lithium-ion battery.
The cathode is insulated from the anode by a separator, which is interposed between the cathode and the anode. In addition, an electrolytic solution is used for the migration of lithium ions. The electrolytic solution, which is an ionic conductor, is prepared by dissolving a lithium salt in an organic solvent. The organic solvent is general by a mixture of an organic solvent having a high dielectric constant and an organic solvent having low viscosity. The organic solvent with a high dielectric constant consists of a cyclic carbonate, and the organic solvent having low viscosity consists of a linear carbonate.
A lithium rechargeable battery containing the organic electrolytic solution is sealed in a stainless steel container, a pouch made of aluminum or the like in order to prevent the leakage of the organic electrolytic solution. Therefore, the cathode, anode, separator, and organic electrolytic solution become hermetically sealed from the outside. Accordingly, if gas is generated in the battery, the pressure inside the battery increases and swelling occurs.
In general, the generation of gas is observed at the interface between the electrodes and the electrolytic solution. If the electrode where gas is produced is the anode, the gas is primarily generated during the manufacturing process. If the electrode where gas is produced is the cathode, gas may be generated when consumers finally use the battery. In particular, if the battery swells due to the generation of gas, electronic devices such as mobile phones powered by the battery can be damaged.
Gas can be generated at the interface between a cathode and an electrolytic solution for various reasons. An increase in temperature is the main cause for the gas generation in a battery and this phenomenon is more easily observed when the battery is charged. In particular, if mobile phones are placed inside automobiles, the surrounding temperature therein can increase to around 100° C. Even in this situation, batteries must retain their performance without swelling.
In order to solve this problem of swelling of batteries due to increased surrounding temperatures, Japanese Laid-open Publication No. 2001-297764 suggests a cathode active material in which the amount of carbonate no more than 0.15% by weight, and the concentration of water no more than 300 ppm. The patent suggests that the reason for limiting the amount of carbonate and the concentration of water lies in the following mechanism. First, carbonate, in particular, lithium carbonate may subject to thermal decomposition, producing CO2, and water reacts with LiPF6 to form HF. Then, the HF may induce gas generation by decomposing carbonate. However, in general, lithium carbonate decomposes at 1310° C. Therefore, it is certain that lithium carbonate is less likely to thermally decompose in lithium batteries. In addition, the suggestion that water reacts with LiPF6 to form HF and that the HF induces the gas generation by decomposing carbonate again, cannot explain the phenomenon in which batteries at high temperatures are more easily observed to form gas when batteries are charged. Moreover, batteries still swell even in situations where the amount of carbonate and the concentration of water are controlled as described above.